This project seeks the structural bases for observed interactions between lipooligosaccharides (LOS) of Neisseria gonorrhoeae and human cell-membrane-expressed and circulating glycoproteins and glycolipids. LOS are the principal outer membrane glycolipids of Neisseria and one of their outermost surface organelles. Each strain makes between one and six physically and antigenically distinct molecules. Heterogeneity reflects physicochemical differences in their glycan moieties. OS of gonoccocal LOS are triantenary molecules that terminate in lactosamine-like structures thatare immunochemically similar to those that terminate the analogous membrane glycolipids of human hematopoetic cells. Antigenic expression within OS is quite complex, and the relationship between chemical and antigenic structures remains obscure. A population of gonoccocal cells consists of different variants, or clono-types, each of which makes a predominant LOS, and that interconvert at a high frequency (10-3). One variant is associated with disseminated disease. Other variants that share structures and epitopes with human glycosphingolipids are associated urethral disease. During the next five years we will explore the biosynthesis and clonal regulation of LOS expression, develop a more complete understanding of the structure and biology of epitopes they share with human cell-membrane glycolipids, and identify the LOS structures of serum sensitive gonococci that bind bactericidal IgM in human sera. We will continue to rely on physical chemistry techniques, particularly mass spectrometry: their development and application to glycolipid research provides a unifying focus. We will also develop and use FACS to analyze dynamic events within the membranes of single cells.